1. Field of the Invention
This invention relates to a method and apparatus for restricting a rotational moment about a longitudinal axis of surface-mount (SMT) connectors, and particularly to a method and apparatus for restricting the rotational moment about the longitudinal axis of SMT DIMM Sockets and other SMT connectors.
2. Description of Background
In computer systems such as personal computers, a socket is referred to as an electrical connector generally mounted on a motherboard (main board) in order to connect extension boards such as extended interface boards for peripheral devices or extended memory boards to the motherboard. The motherboard and extension boards can be electrically connected by plugging the extension boards into the electrical connector.
The structure of a common electrical connector will be described here with the example of an electrical connector used to connect an extension memory module (hereinafter, “module”) referred to as a DIMM (dual in-line memory module) as illustrated in FIGS. 1 and 2. This module corresponds to the extension board described above.
A dual in-line memory module (DIMM) is more and more popular for use in the present PC industry, and thus uses a DIMM socket connector mounted on the motherboard for mechanical and electrical interconnect of the corresponding DIMM therein for signal transmission between the motherboard and the DIMM. A main feature of the typical DIMM connector as illustrated in FIGS. 1 and 2 is that the DIMM connector 10 includes generally a pair of latch/eject members 12 at its two opposite ends so that such DIMM may not only be properly retained in the DIMM connector 10 without possibility of inadvertent withdrawal by vibration or external impact, but also easily ejected from the DIMM connector 10 by rotational movement of the latch/eject member 12.
With more of the industry moving to SMT (Surface Mount Technology) connectors due to PCB wiring density, path length, and electrical signal integrity concerns, new mechanical requirements emerge due to the delicate SMT interface, compared to the more mechanically robust compliant pin and pin-through-hole interfaces in previous applications. This disclosure addresses the forces and strains incurred at the SMT solder joint and pad interface due to rotation about the long axis of an SMT DIMM socket or housing 14, for example, as well as the possibility of pad delamination at the card surface, by minimizing the overall rotation about the longitudinal axis of the SMT DIMM socket, as illustrated in FIG. 2.
Rotation about the longitudinal axis of the SMT DIMM socket 14 is caused by a number of factors. One factor is the amount and location of the center of mass of the DIMM module (not shown). The module acts as a cantilevered beam when assembled into the socket 14, and, where shock, vibration, and dead load effects can all contribute to moments being applied to the DIMM connector 10, particularly when the DIMM module is plugged parallel to the ground and perpendicular to a motherboard 16 on which the DIMM connector 10 is surface mounted thereto. Another factor is due to the design of the connector 10 itself, allowing rotation of the DIMM module upon insertion. The traditional DIMM socket allows approximately 10 degrees of rotation centered about a perpendicular plane to a printed circuit board (PCB) surface defined by the motherboard 16. This allowable rotation, coupled with the high insertion forces required to mate the interface between the DIMM module and the socket, results in a high lateral load forming a torsional moment about the longitudinal axis of the connector inducing an undesirable shear stress to the SMT joint and PCB pad, regardless of orientation of the module and connector with respect to gravity. This stress to the SMT joints, as well as the SMT pad, creates a reliability concern, and the possibility of pad delamination.
Previous designs were mechanically anchored to the PCB via the pin-through-hole or compliant pin nature of the PCB leads, as discussed above which provided a larger reaction force to the lateral shear and torsional moments than the present SMT joints provide. With the present surface-mount design, the reaction forces are carried through the SMT joints and PWB solder pads, which are not as robust as pin-in-hole connections to withstand such forces, and pose a reliability concern.